1. Field of the Invention
This invention relates to a magnetoresistance element for reading the magnetic field intensity of magnetic recording media as signals, especially a magnetoresistance element capable of reading a small magnetic field change as a greater electrical resistance change signal and a magnetic multilayer film suitable for use therein. The term "magnetoresistance" is often abbreviated as MR, hereinafter.
2. Prior Art
There are growing demands for increasing the sensitivity of magnetic sensors and increasing the density of magnetic recording. Active research works have been devoted for the development of magnetoresistance effect type magnetic sensors (simply referred to as MR sensors, hereinafter) and magnetoresistance effect type magnetic heads (simply referred to as MR heads, hereinafter). Both MR sensors and MR heads are designed to read out external magnetic field signals by detecting changes in the resistance of a reading sensor portion formed of magnetic material. The MR sensors have the advantage of high sensitivity and the MR heads have the advantage of high outputs in high density magnetic recording since the reproduced output does not depend on the relative speed of the sensors or heads to the recording medium.
Conventional MR sensors of magnetic materials such as Ni.sub.0.8 Fe.sub.0.2 (Permalloy) and NiCo utilizing anisotropic magnetoresistance effect offer only an MR ratio .DELTA.R/R as small as 2 to 4% and are short in sensitivity as reading MR heads adapted to accommodate for ultrahigh density recording of the order of several GBPI.
Attention is now paid to artificial superlattices having the structure in which thin films of metal having a thickness of an atomic diameter order are periodically stacked since their behavior is different from bulk metal. One of such artificial superlattices is a magnetic multilayer film having ferromagnetic metal thin films and antiferromagnetic metal thin films alternately deposited on a substrate. Heretofore known are magnetic multilayer films of iron-chromium, nickel-chromium and iron-manganese types as disclosed in Japanese Patent Application Kokai (JP-A) No. 189906/1985. Among them, the iron-chromium (Fe/Cr) type was reported to exhibit a magnetoresistance change in excess of 40% at cryogenic temperature (4.2K ) (see Phys. Rev. Lett., Vol. 61, page 2472, 1988). This artificial superlattice magnetic multilayer film, however, is not commercially applicable as such because the external magnetic field at which a maximum resistance change occurs (that is, operating magnetic field intensity) is as high as ten to several tens of kilooersted (kOe). Additionally, there have been proposed artificial superlattice magnetic multilayer films of Co/Cu and Co/Ag, which require too high operating magnetic field intensity.
Under these circumstances, a three-element magnetic multilayer film having two magnetic layers having different coercive forces deposited through a non-magnetic layer was proposed as exhibiting a giant MR change due to induced ferrimagnetism. For example, European Patent Application No. 0483 373 proposed such a magnetic multilayer film in which two magnetic layers disposed adjacent to each other through a non-magnetic layer have different coercive forces (Hc) and a thickness of up to 200 .ANG.. Also the following reports are known.
(a) T. Shinjo and H. Yamamoto, the Journal of the Physical Society of Japan, Vol. 59 (1990), page 3061
Co(30)/Cu(50)/NiFe(30)/Cu(50)!.times.15 wherein the number in the parentheses represents the thickness in angstrom of the associated layer and the number after "x" is the number of repetition (the same applies hereinafter) produced an MR ratio of 9.9% at an applied magnetic field of 3000 Oe and about 8.5% at 500 Oe.
(b) H. Yamamoto, Y. Okuyama, H. Dohnomae and T. Shinjo, the Journal of Magnetism and Magnetic Materials, Vol. 99 (1991), page 243
In addition to (a), this article discusses the results of structural analysis, changes with temperature of MR change and resistivity, changes with the angle of external magnetic field, a minor loop of MR curve, dependency on stacking number, dependency on Cu layer thickness, and changes of magnetization curve.
(c) EP-A1 0483 373/1991
Disclosed is a magnetic multilayer film having two magnetic thin films having different coercive forces stacked through an intervening non-magnetic thin film. An exemplary structure includes a Ni--Fe film of 25 .ANG. or 30 .ANG. thick, an intervening Cu film, and a Co film of 25 .ANG. or 30 .ANG. thick.
(d) EP-A1 0498 344/1992
Disclosed is a magnetic multilayer film having stacked through an intervening non-magnetic thin film two magnetic thin films of (Ni.sub.x Co.sub.1-x)x'.sup.Fe 1-x' and (Co.sub.y Ni.sub.1-y)z.sup.Fe 1-z wherein x=0.6-1.0, x'=0.7-1.0, y=0.4-1.0, and z=0.8-1.0. An exemplary structure includes a non-magnetic thin film of 50 .ANG. thick intervening between two magnetic thin films of 30 .ANG. thick.
(e) Hoshino, Hosoe, Jinpo, Kanda, Tsunashima and Uchiyama, Proceedings of Magnetics Research Meeting of the Japanese Electrical Society, MAG-91-161
This is a confirmation test of (a) and (b). Included are tests of dependency on Cu layer thickness and dependency on NiFe layer thickness. Also reported is the result about the dependency of coercivity of Co on Cu layer thickness which is simulatively determined from the magnetization curve by extrapolation. Magnetization curves are derived from NiFe(30)-Cu(320) and Co(30)-Cu(320) and synthesized for comparison with a magnetization curve of NiFe(30)-Cu(160)-Co(30)-Cu(160). Since the thickness of the Cu intermediate layer is different from that of a three-element multilayer, direct comparison of squareness ratio and coercivity is impossible.
FIGS. 7 and 8 of this article show the dependency on NiFe layer thickness t.sub.1 and Co layer thickness t.sub.2 with the non-magnetic an MR ratio available when the NiFe layer thickness t.sub.1 is varied from 5 .ANG. to 50 .ANG. with the Co layer thickness t.sub.2 fixed at 30 .ANG.. Plotted in FIG. 8 are data at coordinates (t.sub.1, t.sub.2)=(20, 10) and (20, 5). All these data show MR ratios below 4%.
(f) Okuyama, Yamamoto and Shinjo, Proceedings of Magnetics Research Meeting of the Japanese Electrical Society, MAG-91-242
This article describes the phenomenological analysis on giant MR changes by induced ferrimagnetism. With the rotation of magnetic moment of an NiFe layer with low Hc, MR similarly changes. A giant MR phenomenon develops due to the artificially created spin anti-parallelism. It is proven by a difference in MR by an angular change of the applied magnetic field that this phenomenon is different from the anisotropic MR effect of NiFe or the like.
(g) Miyauchi, Araki and Narumiya, the Journal of Japanese Applied Magnetics Society, vol. 17, page 365-368, Apr. 1, 1993
This article of ours was published after the filing date in Japan of the basic application, but before the filing date of this application in the U.S. It relates to a three-element system magnetic multilayer film using NiFe and Co. FIG. 8 of this article shows an MR curve of Cr(50) Cu(50)-Co(10)-Cu(50)NiFe(10)!.times.10 over the magnetic filed range between -10 Oe and +10 Oe. Plotted in FIG. 7 of this article are data at coordinates (t.sub.1, t.sub.2)=(10, 10), (20, 10), (30, 10), (30, 15), and (20, 20) wherein t.sub.1 is a NiFe thickness and t.sub.2 is a Co thickness. Also plotted in FIG. 3 of this article are data representative of an MR slope at coordinates (t.sub.1, t.sub.2 ) =(10, 10), (15, 10), (20, 10), and (30, 10). In all samples, the Cu layer has a thickness of 50 .ANG..
As mentioned above, three-element magnetic multilayer films exhibit a giant MR ratio of about 10% under an applied magnetic field of up to about several hundreds of oersted, though the magnitude of MR ratio is small as compared with Fe/Cr, Co/Cu and Co/Ag. It is to be noted that these disclosures refer to only MR changes under an applied magnetic field of up to about 10 to 100 Oe.
For practical MR head material to find use in ultra-high density magnetic recording, an MR curve under an applied magnetic field of 0 to about 40 or 50 Oe is critical. The above-mentioned prior art three-element artificial superlattices, however, failed to increase MR changes under zero magnetic field, with an MR change approximate to 0. An increase of MR change becomes maximum at about 60 Oe and an MR ratio of about 9% is then obtained. This implies that the MR curve has a slow rise. In the case of Permalloy (NiFe), the MR change has a gradient of approximately 0 across zero magnetic field and the MR ratio remains substantially unchanged, with a differential value of MR ratio being close to 0.
For overcoming such properties, NiFe is provided with a shunt layer having low resistivity such as Ti for providing a shift of the operating point. In addition to the shunt layer, a soft film bias layer of soft magnetic material having a high resistivity such as CoZrMo and NiFeRh is provided for applying a bias magnetic field. The structure having such a bias layer, however, is complex in steps, difficult to provide stable properties, and increased in cost. It also invites a lowering of S/N since it uses a relatively moderate portion of the MR curve.
Moreover, MR heads have a complex layered structure, require heat treatment such as baking and curing of resist material during patterning and flattening steps, and must tolerate temperatures of about 250.degree. to 350.degree. C. Conventional three-element artificial superlattice magnetic multilayer films, however, degrade their properties during such heat treatment.
Also, the prior art three-element system magnetic multilayer films have unsatisfactory properties since their MR slope does not exceed 0.15% /0e over the magnetic field range between -50 Oe and +50 Oe. The only exception is our article (g) referred to above which reports in FIG. 8, a linear MR change over the magnetic field range between -10 Oe and +10 Oe.
Further, it is of importance for MR heads to show an MR curve having a reduced maximum hysteresis width. Although a maximum hysteresis width of 10 Oe or less is recommended in practice, most prior art three-element magnetic multilayer films fail to meet the requirement. The only exception is our article (g) referred to above which reports in FIG. 8, a noticeably reduced hysteresis width.
Often MR heads are used in a magnetic field with a high frequency of 1 MHz or higher for reproduction of high density magnetic recorded signals. Most prior art three-element magnetic multilayer films are difficult to produce an MR slope of 0.03%/Oe or more in a magnetic field with a 1-MHz or higher frequency partly because of their film thickness combination.